Add support for --all to nix build

[lisanna-dettwyler]

Resolves #7165

TODO:

• Add test
• Refactor common logic out of completeFlakeRefWithFragment() so we're not abusing a completion function
• Accept argument to --all which is the fragment to apply to the flakes, e.g. .devShells rather than just getting default completions

Is it OK to abuse RawInstallables as specifying flakes to build --all from? I think so, because normally a flake installable arg evaluates to flake#.packages.<system>.default so there is precedent for this.

By doing it this way, it automatically does the right thing for multiple out links.

Motivation

This is a very popular requested feature.

Context

This is a WIP POC that I will refine.

#7165

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[edolstra]

It probably shouldn't be named --all because that already means "every store path in the Nix store" in commands like nix path-info.

It would probably be best to wait until we have flake schemas (DeterminateSystems#217), because then we have a non-ad-hoc way of querying all the buildable flake outputs.

Another consideration: we do already have a way to build all flake outputs, namely nix flake check. However, it doesn't create symlinks to the outputs.